The present invention concerns a method of manufacturing helical springs from steel wire, whereby the springs' skin is thermomechanically hardened Helical springs of this genus are employed especially in suspensions in the automotive industry, where they must be able to support heavy loads.
Essentially, two basic methods of manufacturing helical springs from steel wire are known--winding and coiling.
Winding begins with already heat-treated steel wire,
Coiling uses untreated wire, which is heated, coiled hot, and finally heat-treated. Coiling is described for instance in Warmgeformte Federn, 52nd International Automobile Exposition (IAA), Frankfurt-am-Main, 1987.
Less known is a third method, whereby the untreated starting material is wound cold, and the spring subjected to heat treatment in a subsequent step.
In coiling, the steel rod is treated by heating, cooling, and annealing. It is usually heated while traveling through furnaces heated by gas or oil. The steel is heated fairly gradually to austeniting temperature and allowed to harden after coiling.
Once hardened and annealed,the springs are preferably air-cooled and then hot set. "Hot setting" in the present context means stressing them at high temperature beyond their flow threshold. It is intended to establish enough inherent stress in the wire to contribute to the springs' static and dynamic load resistance and to improve relaxation and reduce creep.
The hot-set springs are then shot peened to strengthen the wire skin and provide inherent compression. Inherent compression is a particularly effective way of increasing the springs' dynamic strength in that it counteracts any high tensions that may occur at the surface of the wire while the spring is subject to load.
German 3 633 058 C1 suggests improving the steel's mechanical properties by "thermomechanical treatment" of the wire. Thermomechanical treatment differs from the conventional treatment comprising hardening and annealing by the additional step of heating to austeniting temperature followed by plastic deformation of the steel by twisting and/or rolling it.
Also known, from German 4 330 832 C2 is a method of manufacturing helical compression springs that involves shot peening the springs twice.
Eckehard Muller, finally, points out, in "Spannungsstrahlen von Schraubendruckfedern", Draht, 1, 2 (1994), that springs shot peened twice, first stressed and then unstressed, are as good as, but require less material and weigh less, than springs that have not been shot peened at all.
Although the known methods of manufacturing helical springs have been proven, they are not up to producing springs in accordance with the ever stricter demands of the automotive industry in particular for smaller springs that will weigh less and take up less space.